Conventional golf balls have primarily two functional components: the core and the cover. One purpose of the core is to be the “spring” of the ball or the principal source of resiliency, and the core may be either solid or wound. The primary purpose of the cover is to protect the core. Multi-layer solid balls include multi-layer core constructions, multi-layer cover constructions and combinations thereof.
Two-layer solid balls are made with a single, solid core. This single core is typically constructed from a cross-linked rubber, for example polybutadiene, and is encased by a single layer of hard cover material. Increasing the cross-link density of this core material increases the resiliency of the core. As the resiliency increases, however, the compression may also increase, resulting in a stiffer ball and increasing the spin rate of the ball. Spin rate is an important characteristic of golf balls for both skilled and recreational golfers.
Traditionally, in the multi-cavity compression molding of a golf ball cover or inner layer, preforms (preps) are made with much larger volume than required by the core geometry. This standard practice in general assures sufficient material to mold the core and formation of adequate matrix to hold parts together for easy de-molding at the end of the cycle. These molding processes consist of low pressure cures to pre-heat the prep once the prep assemblies are loaded and held under lower pressure to melt and displace significant amounts of material. Once the low pressure cure step is complete, the press closes at full hydraulic pressure to displace the remaining excess material and form a cover while still continuing to maintain a high temperature set point. The temperature set point typically needs to be high to melt excess material out and maintain the sizes especially at the poles. The melt flow variation due to climate changes and material specification changes are routine issues, with the molding parameters, especially temperature set points being constantly adjusted to maintain correct pole sizes. Also voids, longer cycles, frequent mold release applications are typical problems.
A major problem arises in molding a thin cover over a large and/or soft core because temperature and pressure are exerted over a set time independent of what material is placed in the press, or what temperature the material is at when placed in the press, or the actual compression of the core when the cover is formed around it. This may result in variations of the final product characteristics such as compression, weight, size, roundness, and layer adhesion.
Compression molding or retractable pin injection molding (RPIM) methods are most commonly used to form ionomer covers around solid or dual polybutadiene (PBD) cores. It is well known in the art that compression molding of a cover over a very soft PBD core causes a “blow out” of the core due to severe deformation of the PBD core during the mold closing. Also, it is very difficult to obtain acceptable roundness of the molded golf ball especially with softer (25-70 PGA) and larger cores (greater than 1.530″). Excessive core shifting is another major issue encountered. Similar issues are experienced with present RPIM technology due to forces resulting from “pinching” of retractable pins to hold softer PBD cores in the center and “hard to control” multi-directional plastic fluid forces during injection will greatly shift PBD cores as well as produce unacceptable out-of-round product.
A well known practice in the production of polybutadiene golf ball cores is a technique called “bumping” to aid in releasing of residual air in the mold cavity. This maneuver is where the mold is clamped for a short time (at least 15 seconds), and then unclamped to allow the residual air to release. This has been a standard practice in compression molding for over twenty years and is described in U.S. Pat. No. 6,838,036.
A method is needed to increase the successful molding of a cover layer over a very large or soft core “without blowing out” or deforming the core.